Field of the Invention
The invention relates in general to an alternating current (AC)-powered light emitting device (LED) arrangement, and more particularly to an AC-powered LED light engine that is capable of producing light output with relatively low flicker while providing the desired high light emitting efficiency and power factor criteria.
Description of the Related Art
So-called AC LED diode products have become popular because one relatively small, flat module can be joined directly to an AC power line and will produce light suitable for many common illumination purposes. The very first such products consisted of LEDs mounted in antiparallel pairs, with as many as forty such pairs connected in series. Typically, a resistor was placed in series with such an arrangement to provide a more constant (and less voltage sensitive) light output. Such a prior art arrangement is shown in FIG. 1. Later, it was found to be more economical to use a string of single diodes in place of the anti-parallel pairs, still with a limiting resistor (or later a current-controlled resistor). In this configuration, a bridge rectifier was also required to be connected between the two ends of the string and the AC power line. This type of prior art arrangement is shown in FIG. 2.
One weakness of such products was that the light was only produced at the peak of the AC power line, when the instantaneous power line voltage exceeded the sum of the forward voltages of the LEDs. This short duration flash, repeated at 120 Hz, could cause stroboscopic effects and for some vulnerable persons might even induce epileptic seizures. Technology was then developed in the form of integrated high voltage switch arrays that were used to sense the AC line voltage, and to cause shorting switches to be closed across suitable numbers of the LEDs so that the sum of the forward voltages of the unshorted LEDs at each point in the power line cycle roughly matched the instantaneous power line voltage. A schematic of a typical prior art product of this kind is shown in FIG. 3. These products have proven popular because they draw a power line current which is roughly sinusoidal in phase with the power line voltage, resulting in power factor and total harmonic distortion (THD) numbers that are pleasing to the utility companies, while driving the LEDs with a net current that is roughly a half sine wave waveform. Therefore, instead of the intense light flash at the peak of each power line half cycle, the light production (which is proportional to the total LED current) rises smoothly to a peak during a power line half cycle and then declines again smoothly to zero before the next power line half cycle.
However successful these products have been, they have not found application in the most demanding applications—such as task lighting, workshop lighting and office lighting. This lack of success is due, at least in part, to the fact that there is still a residual stroboscopic effect in these switch-controlled arrangements. Although not noticeable by most people most of the time, this effect can produce headaches and eye strain if these light sources are used in these task, workshop and/or office light applications (among others). This fluctuation of the light at 120 Hz is often referred to as “flicker” in the lighting industry. The conventional definition of flicker is the fraction of the minimum point in the LED current waveform in terms of the maximum (Imax) and minimum (Imin) current levels in the waveform, thus:flicker=(Imax−Imin)/(Imax+Imin).
This definition is useful for low frequency sinusoidal fluctuations in a waveform. However, defining flicker in this fashion may not relate well to the perception of the human eye when extremely rapid fluctuations are present. The human eye cannot perceive any fluctuations at more than 120 Hz frequency, and even at 120 Hz the perception is marginal. Therefore, when high frequency fluctuations are present, the conventional definition of flicker does not comport to the perception of the human eye. In particular, if tiny notches of 2 msecs or less are taken out of an LED current waveform, the human eye cannot react fast enough to notice them. This is why high frequency pulse width modulation of LED current is used to produce the perception of dimming of LED light.
It is clear from the foregoing that there is a need for an AC-driven LED light engine that can produce light with low flicker to please consumers, while still having all the attractive features of the existing AC LED lights. A clue to the direction from which such a circuit might come was discussed in a paper entitled “A Driving Scheme to Reduce AC LED flicker” by Tan and Narendran, presented at the 2013 SPIE meeting in San Diego. This paper came to the conclusion that flicker could be minimized by the combination of capacitive and resistive drive to LEDs. However, the authors merely describe a parallel combination of a pair of bidirectional LED strings, the activation of one string of LEDs being resistively limited and the activation of the other, parallel string of LEDs being capacitively limited. The Tan et al. paper does not provide a practical engineering solution as to how this could be done.
As described in the Tan et al. paper, the circuit presented will require either twice or four times the number of LEDs as the various configurations of the present invention described hereinbelow. If the AC LEDs proposed by Tan et al. were replaced with LED strings enclosed by bridge rectifiers, fewer LEDs would be required, but the gap between light outputs from each half cycle becomes a major disadvantage. U.S. Pat. No. 8,569,961 issued to Lee et al. on Oct. 29, 2013 presents a circuit based upon both resistive and capacitive coupling, with a principle using cross-coupled capacitors, but again requires twice as many LEDs as the present invention. U.S. Pat. No. 8,552,656 issued to Choi on Oct. 8, 2013 uses capacitors that are charged on positive half-cycles and negative half-cycles, respectively, to drive a single pair of LED arrays (which thus conduct alternately), and connects the LEDs directly to the ac power line with a series diode for each LED array, but is not capable of leveling the LED current. As a result, a significant “flicker” remains.